This invention relates to protection of transformers, and more particularly to a transformer protection system using a computer for processing various data for the protection of a transformer installed in a power transmission system.
A protective relay device based on the differential principle is now widely used for protecting a transformer installed in a power transmission system. This device decides that an internal fault has occurred in the transformer when the vector sum of currents detected at the terminals of the transformer is not equal to zero. However, the vector sum of the currents is not equal to zero immediately after the transformer is connected to the power transmission line by turning on a circuit breaker, and, as a result, the protective relay device based on the differential principle would act to trip the circuit breaker by deciding that an internal fault has occurred in the transformer. A method proposed to deal with such a defect is based on the finding that a magnetizing inrush current flowing through the transformer at the instant of insertion of the transformer to the power transmission system includes a second harmonic in a large amount. According to the proposed method, the circuit breaker is prevented from being tripped by the protective relay device based on the differential principle when the presence of the second harmonic in an amount more than a predetermined value is detected. A protection system resorting to such a method is now most widely adopted in this field.
However, in a modern very large electric power system, frequency components close to the frequency of the second harmonic tend to be normally included in the power supplied by the electric power system. This results in an adverse effect that a proper command for tripping a circuit breaker is not applied in the event of occurrence of an internal fault of a transformer, resulting in an increased possibility of failure to reliably detect the internal fault by a prior art protective relay device based on the differential principle. As far as such a protective relay device composed of an analog circuit is used, it is unable to attain the desired protective function which can accurately detect the internal fault only, and which is not responsive to an external fault or to the turning-on of the circuit breaker. Thus, a protective relay system for a transformer has been developed which is based on an entirely novel operating principle free from the prior art operating principle such as the differential principle. The outline of this novel protection system is such that a numerical formula is provided which simulates the characteristics of a transformer by the use of a digital computer, and voltage and current values actually detected at the terminals of the transformer are put into the numerical formula so as to decide whether or not an internal fault has occurred in the transformer. (In the prior art, current values only have been detected for the purpose of protection.) A digital type protection system as disclosed in JP-A-59-25527 published on Feb. 9, 1984 is an example. The features of the proposed protection system are as follows:
(a) When a differential current computed on the basis of currents detected at the terminals of windings of a transformer exceeds a predetermined detection leve, decision is made as to whether or not the following relation holds: EQU [V]-[l][di/dt]=[0] (1)
or EQU [y][.intg.Vdt]-[.intg.di]=[0] (2)
where [V] is a column vector of terminal voltages of the transformer windings; [i] is a column vector of terminal currents of the transformer windings; [l] is an inductance matrix of self-inductances and mutual inductances of the windings when the iron core of the transformer is magnetically saturated; and [y] is an admittance matrix which is an inverse matrix of [l].
(b) A magnetizing inrush current flows as a result of magnetic saturation of the iron core of the transformer. Therefore, in view of the character of the principle of decision, the equation (1) or (2) holds substantially when the current is substantially composed of the magnetizing inrush component. When, on the other hand, an internal fault occurs in the transformer, the equation (1) or (2) does not hold because, in such a case, at least one of the windings is not sound, and the transformer core is not generally magnetically saturated.
(c) Therefore, the presence of an internal fault is identified only when the result of computation of the left-hand members of the equation (1) or (2) is not equal to zero.
According to the known digital type protection system described above, a magnetizing inrush current and a current attributable to an internal fault can be discriminated from each other irrespective of their current waveforms. Thus, the transformer can be protected with high reliability.
In the case of the known digital type protection system, the inductance matrix [l] or the admittance matrix [y], which is the coefficient in the equation (1) or (2), must be previously computed on the basis of the dimensions of the transformer windings. However, there are the following problems for the detection of the internal fault with high sensitivity:
(a) A slight error occurs inevitably between the actually detected value and the computed value. Thus, even in the case of the magnetizing inrush current, the result of computation of the left-hand members of the equation (1) or (2) is not perfectly equal to zero, and this obstructs the detection of the internal fault with high sensitivity.
(b) Complicated computation formulas are required for accurate determination of the coefficients for various transformers having different winding structures, and this provides a cause of mis-setting the coefficients.
(c) The coefficients cannot be previously determined when, for example, the dimensions of windings of a transformer are unknown.